Generics in Kotlin and Java with nested classes

Question

Can someone explain, please, why i can't do the code below from Java in Kotlin?

Java:

public static <T extends ViewGroup> void doSomething(T viewGroup) {
    T.LayoutParams params = viewGroup.getLayoutParams();
}

Kotlin:

fun <T : ViewGroup> doSomething(viewGroup: T) {
    val params : T.LayoutParams = viewGroup.layoutParams
}

or

fun <T : ViewGroup> T.doSomething() {
    val params : T.LayoutParams = this.layoutParams
}

Kotlin just doesn't see LayoutParams.

Imho, Kotlin does "the right thing". Static methods, fields and classes do not get inherited to other classes, thus T.LayoutParams does not have a well-defined semantics (other than saying "At execution time T is replaces with its upper bound ViewGroup). The "correct" (and more reabable way) would be to use ViewGroup.LayoutParams. — Turing85
– Turing85, Commented Mar 16, 2018 at 7:58
To expand on @Turing85's point a bit, Java is only faking here as well and its T.LayoutParams means just ViewGroup.LayoutParams. If one of subtypes of ViewGroup has its own LayoutParams class defined, it won't be used. — Alexey Romanov
– Alexey Romanov, Commented Mar 16, 2018 at 17:19

muminers · Accepted Answer · 2018-03-16 07:54:04Z

3

Have you tried?

fun <T : ViewGroup> doSomething(viewGroup: T) {
    val params = viewGroup.layoutParams
}

EDIT: Well I tried it and you can't do that:

val params : T.LayoutParams = viewGroup.layoutParams

But you can do that:

val params: T = viewGroup
val par = viewGroup.layoutParams

answered Mar 16, 2018 at 7:40

muminers

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2 Comments

I think that this is the best way of doing this

Yeah, also tried that, but not what i really wanted, Anyway thanks!